Replacing paint on aircraft exterior surfaces with appliqués is desirable for a variety of reasons, including weight reduction, elimination of hazardous materials associated with painting and paint stripping, maintenance cost savings, and lightning strike protection. Appliqués also allow incorporation of drag-reducing riblets and hydrophobia/anti-ice materials and textures.
Appliqués typically include three components: (i) a protective barrier film; (ii) a pressure sensitive adhesive (PSA); and (iii) a release liner. Typical material requirements for appliqués used on aircraft include: a temperature range from around −65 degrees Fahrenheit up to around 230 degrees Fahrenheit; fluid resistance to a wide range of substances such as jet fuel, hydraulic fluid, aircraft wash fluids, de-icing fluids, lube oil, and the like; exposure to environmental conditions such as ultraviolet (UV) radiation, hot condensing humidity, salt spray, rain erosion, wind erosion, and the like; and easy removal and repair.
The PSA plays an important role in an appliqué's ability to withstand such conditions. In addition, and unlike paint systems, ability to maintain bond of an appliqué to an aircraft skin may affect flight safety considerations. For example, it must be demonstrated that an appliqué will not peel in flight as a large section or “gore”—even if peeling initiates due to impact or other damage.
Current PSAs used for aircraft exterior applications generally are acrylic polymer-based adhesives, and can have good moisture resistance and relatively high bond strengths. While some current PSAs are resistant to some aircraft fluids, few PSAs are currently available that can resist all aircraft fluids, can withstand extended exposure to high humidity, and can maintain adhesion over a wide range of temperatures. For example, many acrylic PSAs tend to lose peel adhesion to the protective barrier film at temperatures above 150 degrees Fahrenheit.
A current PSA available from the 3M Company maintains adhesion values over a temperature range of −65 degrees Fahrenheit to 230 degrees Fahrenheit or higher while resisting aircraft fluids such as jet fuel. However, this PSA is unavailable as a transfer film adhesive (which is the form used in several current appliqués). Moreover, cost of this PSA precludes its use as a commercially-viable PSA for commercial aircraft use.
Currently known PSAs with acceptable resistance to aircraft fluids typically lose the bond interface to the protective barrier film as the PSA and the barrier film go through their glass transition temperatures. In such a case, in the event of a fracture at the interface the PSA will remain adhered to the underlying substrate but the barrier film will peel off the PSA. In addition, the PSAs fail cohesively on peeling at a low peel strength at high temperatures, such as temperatures greater than 200 degrees Fahrenheit.
The foregoing examples of related art and limitations associated therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.